Re: [PATCH 5.15 1/2] dma-mapping: remove bogus test for pfn_valid from dma_map_resource

[Georgi Djakov <djakov@kernel.org>]

On 21.04.22 9:43, Greg KH wrote:
> On Wed, Apr 20, 2022 at 05:43:40AM -0700, Georgi Djakov wrote:
>> From: Mike Rapoport <rppt@linux.ibm.com>
>>
>> [ Upstream commit a9c38c5d267cb94871dfa2de5539c92025c855d7 ]
>>
>> dma_map_resource() uses pfn_valid() to ensure the range is not RAM.
>> However, pfn_valid() only checks for availability of the memory map for a
>> PFN but it does not ensure that the PFN is actually backed by RAM.
>>
>> As dma_map_resource() is the only method in DMA mapping APIs that has this
>> check, simply drop the pfn_valid() test from dma_map_resource().
>>
>> Link: https://lore.kernel.org/all/20210824173741.GC623@arm.com/
>> Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
>> Reviewed-by: Christoph Hellwig <hch@lst.de>
>> Acked-by: David Hildenbrand <david@redhat.com>
>> Link: https://lore.kernel.org/r/20210930013039.11260-2-rppt@kernel.org
>> Signed-off-by: Will Deacon <will@kernel.org>
>> Fixes: 859a85ddf90e ("mm: remove pfn_valid_within() and CONFIG_HOLES_IN_ZONE")
>> Link: https://lore.kernel.org/r/Yl0IZWT2nsiYtqBT@linux.ibm.com
>> Signed-off-by: Georgi Djakov <quic_c_gdjako@quicinc.com>
>> ---
>>   kernel/dma/mapping.c | 4 ----
>>   1 file changed, 4 deletions(-)
> 
> I took this, but I do not understand why patch 2/2 in this series is
> needed, as Sasha points out.  Cleanups are nice, but is it necessary
> here?

It's needed as it removes the "select HAVE_ARCH_PFN_VALID" from the arm64/Kconfig.
This will make us use the generic pfn_valid() function in mmzone.h, instead of the
arch-specific one, that we are dropping.

Thanks,
Georgi